The Nuclear Ribosomal Transcription Units of Two Echinostomes and Their Taxonomic Implications for the Family Echinostomatidae.

Echinostomatidae is a taxonomically complex group with substantial species diversity and richness. The vast majority of species in this family parasitize birds and mammals, including humans, causing significant economic losses and medical costs. In this study, (Digenea, Echinostomatidae) and (Digenea, Echinostomatidae) were isolated from domestic duck and , respectively. The nearly complete ribosomal transcription unit (rTU) sequences of two echinostomes were obtained, with the rTU for . being obtained for the first time. The nearly complete rTU sequence of . (6790 bp) and . (6893 bp) encompass the small-subunit (18S) ribosomal DNA (rDNA), internal transcribed spacer 1 (ITS1), 5.8S rDNA, internal transcribed spacer 2 (ITS2), and large-subunit (28S) rDNA. The complete lengths of 18S, ITS1, 5.8S, ITS2, and 28S sequences for . are 1989 bp, 444 bp, 162 bp, 431 bp, and 3858 bp, respectively. For . , complete or nearly complete lengths of these sequences are 1929 bp (nearly complete), 419 bp, 162 bp, 432 bp, and 3848 bp (nearly complete), respectively. The 18S, ITS, and 28S sequences of . show the highest sequence similarity with other . . The ITS and 28S sequences of . show the highest sequence similarity with other . , while 18S sequence shows the highest similarity with . . This is likely due to the unavailability of the 18S sequence of . in GenBank. Repeat sequences were identified in 18S, ITS1, ITS2, and 28S sequences, with the 28S sequence containing the most repeats and the 5.8S sequence having none. The results of phylogenetic reconstruction indicated that . clusters with other spp., while . clusters with other spp., forming sister taxa. This study not only provides the first rTU sequence for . but also reinforces the sister group status of to through phylogenetic evidence. Finally, this study represents the first record of the . as a new host for . and the first report of a bird from the crane family (Gruidae) as a host for any echinostome species. These findings are significant as they expand our understanding of the host range and ecological interactions of Echinostomatidae. The data obtained provide a valuable resource of molecular markers for studying the taxonomy, population genetics, and systematics of the family Echinostomatoidea. This research contributes to a more comprehensive understanding of the evolutionary relationships and biodiversity within this complex group of parasites, which is crucial for developing effective strategies to mitigate their impact on both wildlife and human health.